The improvement of current antiviral vaccines and the development of novel vaccines depends on improving our understanding of viral attachment and fusion glycoproteins. Critical insight into the structure and function of viral glycoproteins is provided by studying their biosynthesis in virus- infected cells. We have studied the biosynthesis of influenza virus hemagglutinin. The HA is a critical antigen for protecting individuals against influenza, which remains a major cause of morbidity and mortality nationally and internationally. In addition, the fact that the HA is the best characterized viral glycoprotein allows it to serve as a model for elucidating general features of viral glycoprotein structure, function, and antigenicity. In the past year we have continued our studies on the biosynthesis of the influenza HA. Like many integral membrane glycoproteins, the mature HA is an oligomer, consisting of three identical monomeric subunits. We have used genetic and immunocytochemical techniques to determine where oligomerization occurs. Our finding are consistent with the idea that HA trimerization occurs in the intermediate compartment that exist between the endoplasmic reticulum and the Golgi complex. We have also continued our studies on the mechanism of action of brefeldin A (BFA). BFA is a fungal metabolite that blocks the exocytosis of secretory and integral membrane proteins. BFA was chemically coupled to a fluorescent compound (Bodipy) and the subcellular distribution of BFA-Bodipy determined by fluorescent microscopy. This revealed that BFA binds to membranes, and appears to have a particular affinity for the endoplasmic reticulum.